The invention relates to a substrate for a lithographic printing plate made of rolled sheet of an aluminum alloy containing up to 2% iron. Also within the scope of the invention is a process for manufacturing such a substrate.
Lithographic printing plates are as a rule made up of an aluminum sheet which bears a light-sensitive layer. This sheet must satisfy a series of requirements. The printing plates, stretched over rotating cylinders must exhibit high yield strength and high fracture strength, and must not fail mechanically after a long service life of several hundred thousand passes. The mounting conditions demand high ductility. Any baking of the light-sensitive layer exposes the aluminum sheet to temperatures of 220.degree. to 270.degree. C.; also after such a treatment the sheet must exhibit sufficiently high static and fatigue strength values. The sheet surface is roughened mechanically, chemically or electrolytically in order to ensure adequate water flow during printing, to achieve a uniform surface of low reflectivity and also to provide a suitably adhesive base for the light-sensitive layer. This roughing must not allow pores that are too coarse to be formed as that would lead to erroneous take-up and release of printing fluid. The surface of the sheet must therefore be free of coarse precipitated intermetallic particles and also free of coarse agglomerates of fine particles as extraction of these during roughing would leave correspondingly coarse pores behind.
Conventional materials for printing sheet are the aluminum alloys AA 1050 (with 99.5% purity Al), AA 1200 (with 99.2% purity Al) and AA 3003 (containing at least 1% Mn). The alloys AA 1050 and AA 1200 do indeed exhibit a good surface finish, but have static and fatigue strength values that are too low for many printing plate applications; the alloy AA 3003 on the other hand exhibits high strength values but because of coarse precipitates and clusters of precipitates gives rise to problems when high quality finishes are required.
Alloys with higher iron contents have also been proposed, for example in patent EP-A 67 056 an aluminum alloy containing at most 1.2% Fe, the rest aluminum and impurities each at most 0.15%. In that document one is advised to avoid higher iron contents as these lead to harmful, coarse precipitates; for a further strength increase however, one is recommended to employ for the printing plate a laminate in which the mentioned alloy is to be used simply as the cover sheet along with a core material of any other aluminum alloy of higher strength. Known from the Japanese patent JP-A-52 029 301 is an aluminum alloy for lithographic printing plates, containing 0.6 to 2% Fe, at most 0.15% Si, if desired at least 0.5% Mg, the rest aluminum and trace amounts of impurities. As ingots continuously cast from melts of this composition exhibit Al.sub.3 Fe particles on the surface and Al.sub.6 Fe particles in the ingot interior, the Al.sub.3 Fe however being in a coarser and less favourable form than the Al.sub.6 Fe particles, these ingots have to be scalped deeply.
The object of the present invention is therefore to prepare a substrate for lithographic printing plates out of rolled sheet made from an aluminum alloy which satisfies the above mentioned high demands with respect to mechanical properties, surface characteristics and etching behaviour, without requiring expensive additional operations. Further, a process which leads to such a substrate should be developed.